Hydrogen fuel cells have been proposed utilizing a reactive chemical hydride as a fuel source. Normally the chemical hydride is reacted with water to liberate hydrogen gas which then is consumed by the fuel cell. Other protonic solvents besides water (e.g. alcohols, organic acids) may also be employed. Some chemical hydrides currently used include CaH2, NaBH4 and LiAlH4. Other reactive metal hydrides which could also be used are NaAlH4, KAlH4 and MgAlH4.
In such a fuel cell system, a housing is often employed to contain the fuel and reaction products in order to protect the fuel from reacting with atmospheric moisture when the fuel cell is not in service and to collect the generated hydrogen so it can be conducted to the fuel cell. It is desirable to include a maximum quantity of fuel in a housing of a given volume in order to achieve the highest possible hydrogen output and the longest possible running time for the fuel cell. However, the disadvantage of packing fuel too tightly into the housing is that the access of water to some parts of the fuel becomes restricted and the rate of hydrogen generation is then limited by the slow diffusion of water into the bulk of the fuel. As solid reaction products accumulate they may further restrict water access, choking off the reaction. Ultimately the reaction may reach a standstill and fuel utilization will be incomplete.
Typically the chemical hydride fuel is compressed into cylindrical pellets and these are then packed or recompressed into a cylindrical fuel holder. The fuel holder may be the same container as the housing or the fuel holder may be a separate structure which is located inside the housing. The fuel holder may contain perforations to permit water ingress through the fuel holder walls into the outer surface of the pellet, from where water can then diffuse radially inward through the pellet thickness. The fuel pellet itself may have radial holes or channels to facilitate water permeation into the depth of the pellet. Alternatively, the fuel pellet may have a hollow cylindrical core to permit water permeation into the hollow core from where the water diffuses radially outward, through the pellet thickness. Both of these pellet configurations may be combined so that water diffusion can occur simultaneously inwardly and outwardly along the pellet radius. But, as the pellet radius is increased, it becomes more and more difficult for water to reach the interior portions of the pellet which lie at some distance from the inner or outer surfaces.
To overcome such problems, the pellet density inside the fuel container may be decreased, leaving open voids and connected channels for water diffusion. This can be accomplished by simply loading loose powder or small chunks of chemical hydride into the fuel container and leaving sufficient void space. Porosity builders such as zeolites, perlites or hollow fibers may be mixed with the hydride powder and then the mixture may be compacted to form pellets.
Both of these approaches create voids and channels which can improve utilization efficiency and the hydrogen production rate. However these approaches inevitably reduce the total quantity of chemical hydride contained within a housing of a given volume and consequently the quantity of hydrogen which can be generated from that fuel.